Plus and Minus Signs

Date: 08/14/2002 at 17:38:32
From: Ozgur Cobanoglu
Subject: Plus or minus sign '±'
I came across two signs while I was trying to do something with
Microsoft Equation Editor. One of them is ± and in the other one the
minus is over the plus sign. There must be a difference because there
are two different signs. I have heard that one of them comes from
ancient Egypt, but I don't know which.
I want to learn their history and the difference in usage.
Thank you.

Date: 08/14/2002 at 22:43:19
From: Doctor Peterson
Subject: Re: Plus or minus sign '±'
Hi, Ozgur.
We usually use the plus-or-minus sign, with the plus on top, when we
want to indicate that both signs are valid. The classic example is in
the quadratic formula, where each choice of sign for the square root
leads to a different root.
In some cases there will be two places in an equation (or set of
equations) where a choice of sign is needed, and the choice is
linked. If when one is chosen as positive the other is to be
negative, we use a minus-or-plus sign for the second. This indicates
that we choose either the top sign in both plases, or the bottom sign
in both places. This is illustrated here in Eric Weisstein's
MathWorld:
http://mathworld.wolfram.com/PlusorMinus.html
As for its origin, Jeff Miller's page
Earliest Uses of Symbols of Operation
http://jeff560.tripod.com/operation.html
says
Plus-or-minus symbol (±) was used by William Oughtred
(1574-1660) in Clavis Mathematicae, published in 1631
(Cajori vol. 1, page 245).
There may have been a similar symbol in Egypt, but it certainly would
not have had this meaning.
If you have any further questions, feel free to write back.
- Doctor Peterson, The Math Forum
http://mathforum.org/dr.math/